1. Field of the Invention
The present invention is related to the use of compressor systems (e.g., turbochargers) for boosting the power output of combustion engines, and in particular to the use of recirculated compressor discharge air to facilitate the operation of such compressors.
2. Description of Related Art
Vehicle engine turbochargers provide an advantageous boost to engine power, especially at higher engine speeds. A turbocharger uses the exhaust gasses from an engine to drive a turbine that drives a compressor, which, in turn, increases the pressure of the engine intake air. The compressed engine intake air results in the boost of engine power. In some turbocharger engine systems, a portion of the exhaust gases from the engine is recirculated back to the intake of the engine to control emissions. In other systems, the compressor drive may be electrically assisted or the turbine may include a variable geometry nozzle to further increase performance.
In certain circumstances, a turbocharger can experience what is commonly referred to as a “surge” condition. Generally, surge occurs when the compressor is driven into low-flow, high pressure-ratio conditions, the result of which is that the compressor blades are forced to operate at such high incidence angles that significant flow separation occurs on the blades. Surge can result in severe aerodynamic fluctuations within the compressor of the turbocharger and can even cause damage to the engine or its intake pipe system.
In one instance, surge can occur when compressor exhaust gas recirculation flow is relatively high at low engine speed. The remaining engine cylinder volume for fresh air at this point becomes smaller. In order to maintain engine torque and load performance, the boosting pressure needs to increase (thereby increasing air density) to keep the same mass flow rate of fresh air into the engine. As a result, the compressor has to work at a relatively higher load and pressure ratio, while the airflow remains relatively low.
In another instance, surge can occur when a relatively high specific power output (such as 70 to 80 kilowatts per liter) is required of the engine and electrically assisted boosting of the compressor is employed. In still another instance, surge can occur when a quick compressor response is required using electrical boosting and/or the use of variable turbine nozzle vanes. In another instance, sudden deceleration of the engine due to closing of the throttle valve can result in surge.
It would be advantageous, therefore, to reduce the occurrence of surge in a turbocharger. Reducing such occurrence would allow an expanded range of conditions in which the turbocharger would be able to boost engine power.